Composite fluid filters made up of two or more layers of filtering media are known in the art. Included among such filters are ones having at least one layer of a conventionally-prepared nonwoven fabric whose fiber content, at least in part, is made up of synthetic polymer fibers such as nylon (polyamide), polyester (e.g., polyethylene terephthalate), viscose rayon, cellulose acetate, polyolefin, acrylic or alkyl acrylate copolymer fibers. The fibers in these conventional nonwoven fabrics can be bonded together by heat, pressure or a combination of the two to fuse individual fibers together at their crossover points, or by applying to a fibrous mat or web a liquid adhesive substance, e.g., a natural or synthetic polymer latex such as one containing natural rubber, an acrylonitrile-butadiene or acrylonitrile-butadiene-styrene adhesive latex or emulsion, or the like. Mechanical means, e.g., needle punching, can also be used to bond such conventional nonwoven fabrics.
Composite filters in which conventional nonwoven fabrics are adhesively or mechanically bonded to each other or to other filter media are also known. Japanese Kokai No. 55-99315, published Jul. 29, 1980, for instance, discloses nonwoven air filters constructed from at least two layers of such nonwoven fabrics of coarse structure and having:
". . . an embossed pattern wherein a coarse structure and a dense structure are intermittently unitized."
This Kokai also speaks of filters with improved shape retaining properties as disclosed in Japanese Patent Publications Nos. 4316274 and 43-28530, such filters having partially heat fused surfaces, with these fused portions being converted into films having a deeply embossed structure.
U.S. Pat. No. 4,540,625, issued Sept. 10, 1985 to Sherwood, discloses flexible, air-permeable, highly absorbent fiber-particle composites useful as air filters in which a nonwoven substrate or nonwoven fibers are combined with in-situ formed organic polymer fibers and solid sorptive particles; see, e.g., column 2, lines 16-49.
Composite air filters made up of a pleated paper filtering medium and a non-woven filter batt are disclosed in U.S. Pat. No. 4,636,231, issued Jan. 13, 1987 to Thornton et al.; see, e.g., column 1, lines 24-47. Batts "made from fibers which are not thermoplastic but which [are] impregnated with a thermoset binder" are specifically disclosed.
U.S. Pat. No. 4,666,479, issued May 19, 1987 to Shoji, discloses air filters for semiconductor wafer containers which can be composites of one or more "filter films" (e.g., one produced from foamed urethane material) and a nonwoven sheet; see, e.g., from column 2, line 58 to column 3, line 16.
Dust filters made by depositing glass fibers by "papermaking methods" on a nonwoven thermoplastic fiber support are disclosed in U.S. Pat. No. 4,728,349, issued Mar. 1, 1988 to Oshitari; see, e.g., from column 1, line 63 to column 2, line 2. The use of "an adhesive binder" is disclosed at column 4, lines 5-13.
U.S. Pat. No. 4,765,812, issued Aug. 23, 1988 to Homonoff et al., discloses air laid fibrous filter media containing mixtures of small diameter and coarser fibers together with a binder material which can be a thermoplastic or thermosetting resin; see, e.g., column 2, lines 3-13 and from column 2, lines 62 to column 3, line 14.